The performance of gas turbine engines, particularly those used to power fighter aircraft, can be detrimentally impacted by several factors. One of these factors is referred to as "tip clearance", which is the gap between the rotating blades and engine case that surrounds the rotating blades. Overall engine performance is particularly sensitive to tip clearance in the compressor section of the engine.
A certain amount of tip clearance is required to accommodate relative movement between compressor blades and the engine case under engine conditions such as surge, aircraft maneuvers, and differences in thermal expansion between the engine rotor and the engine case during engine acceleration and deceleration which decrease the gap. Gas turbine engines typically include outer air seals which are located in the engine case radially outward of each of the rotors. These outer air seals are usually made of an ablative material that is softer than the material on the tips of the blades, so that if the tip of a rotating blade contacts, or "rubs", the outer air seal, the outer air seal becomes sacrificial and the blade tip sustains little or no damage.
While outer air seals provide protection against blade damage and wear, when a blade tip rubs and grinds away part of the outer air seal, tip clearance increases. Unfortunately, as tip clearance increases, engine performance decreases. Over time, the accumulation of compressor blade tip rubs against the outer air seals can cause substantial deterioration of engine performance.
What is needed is a compressor blade that is capable of multiple rubs with the outer air seal, or the engine case, with no significant increase in tip clearance.